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Wildlifewriter Founder member
Joined: 04 Aug 2005 Posts: 948 Location: Norn Iron
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Posted: Fri Oct 28, 2005 11:34 am Post subject: The Gap in the Sats |
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Do you ever look look at the "Sky View" display on your GPS?
Most units have a section which shows the signal strength from various satellites in view - and some have a little "chart" which shows where the sats are in the sky, relative to your current position. Have you noticed something strange about it?
I have.
Now, most GPS users just take all of this satellite stuff for granted. They're up there, we're down here, this shows the position. Hike completed, or cache found - job done. But it's not that simple...
There are thirty American nav satellites in operation at the moment. (Plus some which are out of service, under test or just loafing around.) On the GPS display, the positions of these seem to be random. They change from hour to hour, but - seen from one place - there's no obvious pattern
The pattern IS there. These GPS satellites are in orbit around the Earth, at an altitude of some 20,500 Km. (10,900 Nautical miles). The orbits are inclined to the equator at an angle of 55º. Like this...
Of course, that diagram doesn't help much because the Earth is a sphere. If we look at the orbits on a projected map, the whole system looks something like...
Notice something interesting: each satellite track (in a rather fetching pink) passes over a different part of the surface - but none of them pass over the North or South Poles. This is a result of the inclined orbits - and it means that none of the satellites are overhead at much more than 60º North, or South.
I don't know about you, but I'm still having some difficulty in visualizing what all this means , as seen from the ground. To help, I set a little program running on my PDA, which records the track of each GPS satellite over a 12-hour period. (Because the sats orbit twice in one day, it gives a picture of the whole constellation as it's seen from here.) This is the result...
... which is kinda interesting. The tracks show that in our part of the world (my latitude is 54º 33' N) there is an area of the sky where GPS satellites are NEVER seen.
I didn't know that...
... And I'll bet YOU didn't, either.
-Wlw. |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Fri Oct 28, 2005 7:05 pm Post subject: |
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The recorded track that your last PDA pic gives indicates the actual track that the satellite takes. If you could see it, looking up, (a bit like a contrail left by a jet) then that would show its path.
That doesn't mean that the satellite isn't in view, as it can "see" a large patch of the ground from where it is but from the side slightly.
Presumably the orbits of the satellites (as set by the US military) allows them to concentrate their view of the surface for military purposes, rather than civil ones. Perhaps, due to the "Special Relationship" they don't need to look at us (UK and NI) too closely as they won't have cruise missiles targeted on Belfast??
In any case, there doesn't seem to be much point in having too many satellites orbiting over the poles, as there is little need for them here. Even with the 55 degree inclined orbit, there will still be at least four satellites visible (but lower on the horizon) even at the poles. In this way, satellite coverage is maximised in areas of high population, below say 60 degrees north / south.
There is a good webpage - http://www.kowoma.de/en/gps/orbits.htm that shows some GPS satellite orbits, and from here, you can go to a Java applet on the NASA site that shows the J - orbital path that satellites take. This applet can be speeded up to show the path of any satellite. |
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Wildlifewriter Founder member
Joined: 04 Aug 2005 Posts: 948 Location: Norn Iron
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Posted: Fri Oct 28, 2005 7:44 pm Post subject: |
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HighlandNick wrote: | The recorded track that your last PDA pic gives indicates the actual track that the satellite takes. If you could see it, looking up, (a bit like a contrail left by a jet) then that would show its path.
That doesn't mean that the satellite isn't in view, as it can "see" a large patch of the ground from where it is but from the side slightly.
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I didn't say that it wasn't in view - only that no satellite crosses that particular patch of sky.
Quote: | Even with the 55 degree inclined orbit, there will still be at least four satellites visible (but lower on the horizon) even at the poles. In this way, satellite coverage is maximised in areas of high population, below say 60 degrees north / south. |
Using Trimble's simulator, with a 10º horizon mask, it seems that there are (on average) rather MORE satellites visible from the North pole at any given hour, than from the equator - but they are, as you rightly point out, lower in the sky.
This is slightly counter-intuitive and I'm not mathematician enough to work out the geometry of why it happens.
-Wlw. |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Fri Oct 28, 2005 8:16 pm Post subject: |
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Wildlifewriter wrote: |
I didn't say that it wasn't in view - only that no satellite crosses that particular patch of sky. |
I take your point, I misinterpreted what you intended!!
Quote: | (Nick) Even with the 55 degree inclined orbit, there will still be at least four satellites visible (but lower on the horizon) even at the poles. In this way, satellite coverage is maximised in areas of high population, below say 60 degrees north / south. |
Quote: | (wlw) Using Trimble's simulator, with a 10º horizon mask, it seems that there are (on average) rather MORE satellites visible from the North pole at any given hour, than from the equator - but they are, as you rightly point out, lower in the sky.
This is slightly counter-intuitive and I'm not mathematician enough to work out the geometry of why it happens.
-Wlw. |
Why you should see more? I would have to ponder this one a bit longer!!
Anyway, with the 55 degree maximum angle from the equator, it leaves a nice little hole - do you remember the Spirograph?? This was a nice way of looking at advancing "orbital" paths in a simple manner. Repetitive circles overlapping and precessing give a circular area with no coverage.
There will be a similar size hole over the south pole as well, as the satellite orbits are also inclined at 55 degrees south.
On a slightly different tack, but no doubt connected somehow, have you had a look at the polar coverage using Google Earth?? |
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Billy Twigger Founder member
Joined: 30 Aug 2005 Posts: 352 Location: N55 51.686 W5 05.647
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Posted: Fri Oct 28, 2005 10:40 pm Post subject: |
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You guys need to stay in more! |
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Naefearjustbeer Founder member
Joined: 20 Sep 2005 Posts: 118
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Posted: Sat Oct 29, 2005 8:47 am Post subject: |
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I managed to glean Blah Blah Blah GPS Blah Blah PDA Blah Blah SKY Blah GAP Blah Blah Blah from that lot _________________ www.naefearjustbeer.co.uk |
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Wildlifewriter Founder member
Joined: 04 Aug 2005 Posts: 948 Location: Norn Iron
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Posted: Sat Oct 29, 2005 11:02 am Post subject: |
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Naefearjustbeer wrote: | I managed to glean Blah Blah Blah GPS Blah Blah PDA Blah Blah SKY Blah GAP Blah Blah Blah from that lot |
Well don't try to read technical articles while the baby's crying, then.
-Wlw. |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Sat Oct 29, 2005 7:37 pm Post subject: |
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A couple of Spirograph images that show the same hole from precessing curves:
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Billy Twigger Founder member
Joined: 30 Aug 2005 Posts: 352 Location: N55 51.686 W5 05.647
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Posted: Sat Oct 29, 2005 10:18 pm Post subject: |
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HighlandNick wrote: | A couple of Spirograph images that show the same hole from precessing curves: |
Results
precessing was not found in the Cambridge Advanced Learner's Dictionary
Did you spell it correctly? Here are some alternatives:
parishes
.
(First and last elements of unreasonably long list - Admin)
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pursues
I think I know what you meant! and admin censored it ! (it was prozac)
Last edited by Billy Twigger on Sat Oct 29, 2005 11:38 pm; edited 1 time in total |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Sat Oct 29, 2005 10:25 pm Post subject: |
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Hah Hah
Try Googling for:
gyroscope precessing |
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Guest
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Posted: Sun Oct 30, 2005 8:13 am Post subject: |
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A satellite directly overhead (90 degrees elevation) is completely useless for navigation as one of the major error components in the system is directly proportional to the tangent (tan) of the angle of elevation, since tan 90 = infinity you get infinate positional error. Running alongside each satellite orbit are large strips of the earth's surface where the signal is usable so the strips of a satellite orbiting at 60 degrees latitude extend to the poles. |
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Billy Twigger Founder member
Joined: 30 Aug 2005 Posts: 352 Location: N55 51.686 W5 05.647
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Posted: Sun Oct 30, 2005 8:50 am Post subject: |
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Billy Twigger wrote: | You guys need to stay in more! |
I apologise for being so flipant - this is a really interesting thread. |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Sun Oct 30, 2005 9:39 am Post subject: |
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Billy Twigger wrote: |
I apologise for being so flipant |
Well I looked up "flipant" in the dictionary and couldn't find it
It suggested i tried:
flip ant
flip-ant
flippant
.. (That's quite enough. For a forum where pointless off-topic posts are welcomed - nay, encouraged - please go HERE -GxAdmin)
Apologies to the admin man who will probably have to delete this list (and me) as well. |
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Wildlifewriter Founder member
Joined: 04 Aug 2005 Posts: 948 Location: Norn Iron
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Posted: Sun Oct 30, 2005 10:06 am Post subject: |
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mckryton wrote: | A satellite directly overhead (90 degrees elevation) is completely useless for navigation as one of the major error components in the system is directly proportional to the tangent (tan) of the angle of elevation, since tan 90 = infinity you get infinate positional error. |
Nonsense.
A satellite directly overhead still contributes an element to the computed 3-D positional solution. Since the solution is based on ranging (not "angles") this contribution is as valid as any other.
Because a minimum of four ranging signals is required for a 3-D solution, there could only be an "infinate (sic) positional error" if all FOUR satellites were to be directly overhead at the same instant.
This cannot happen, therefore such an error cannot occur.
-Wlw. |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Sun Oct 30, 2005 1:00 pm Post subject: |
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Quote: | .. (That's quite enough. For a forum where pointless off-topic posts are welcomed - nay, encouraged - please go HERE -GxAdmin)
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Yes, go on, click on the link. I did - it's pure dead brilliant |
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